Bloggs, If 'they' really dont like you 'they' can try to fail you out of the company. It's not hard to invent a standards "problem" when you check someone, if you want to; No one is perfect. This would occur whether or not a promotion occured by the means of datal or experiential seniority. They only remedy for this situation is to ensure that the industrial award provides for an independant person to come on the jump seat during check rides, if required.
HI- The rostering senario that you speak of would only occur with crewing department complicity. Given that the 100hr/month pilot would soon run into duty limits, the system would self correct quite quickly. Rostering systems whereby datal seniority plays role in bidding for flying blocks can cause the same problems too, if 'senior' pilots are bidding for reserve blocks. Same goes for time off at Christmas and new year. There are various practices that companies without datal seniority use that ensures that a fair roster is generated for everyone.
Quite frankly, I am not sure that it is desirable that every pilot should get a shot at upgrading. I am sure that everyone knows a Captain or two that shouldn't be in the job. Consider what would happen if a pilot who was marginal was promoted and then killed people; you could make his epitath "Well he was next in line and he did pass the check". If it came out at an inquest that this person was not the most suitable for promotion at the time, how would you then defend the concept of datal seniority to the greiving relatives, the insurures, the legal system, the industry or yourself?